Edge Loop creates duplicate in 2.8
It looks like, for some reason, the vertices at the bottom of the top cap and the top of the bottom cap have been duplicated -- so there's a "hidden" set of faces in the middle, which you're exposing when you bend the other set out of the way.

If I'm right, fixing it will be easy. There are two options:


If you can still undo the "extrusion": in Edit Mode, select all of the vertices with A. Then hit Alt-M, and click "By Distance". This will automatically merge all vertices that are within a certain very small threshold from one another, removing duplicates. After you've done that, you should find that the loop cut step works as expected.
If you've already committed to the "extrusion": Select the extra faces (the ones that weren't affected by your loop cut) and delete them with X. Then do the same as I described above to merge the extra vertices.